1. Field of the Invention
The present invention relates to an ultrasound imaging system. In particular, the invention relates to an ultrasound imaging system based on a pulse compression technique using modified Golay codes.
2. Description of the Related Art
Conventionally, a medical ultrasound imaging system obtains information about a human body by transmitting short ultrasound pulses. FIG. 1 shows a block diagram of a conventional short-pulse ultrasound imaging system 100, which comprises a transducer array 1, a pulser 11, a TX(transmission) focus delay memory 14, a TX/RX(receiving) switch 21, a receiver 31, a beamformer 35, an RX focus delay adjuster 36, a signal processor 41 and a scan converter 42.
The TX focus delay memory 14 stores a delay pattern of ultrasound pulses to be transmitted into a human body from the transducer array 1. The TX focus delay memory 14 provides a binary sequence corresponding to the delay pattern to the pulser 11.
As a method of determining the TX focus delay for each transducer, a fixed-focusing technique is commonly used, which focuses the energies of the ultrasound pulses on a predetermined point inside the body. Recently, as one of efforts to resolve the problem of limited resolution due to transmitting with the fixed-focusing while receiving with the dynamic focusing, a synthetic aperture technique has been studied. With the synthetic aperture technique, one or more transducers can be used for transmitting ultrasound and bi-directional dynamic focusing for transmitting and receiving is possible. By using the synthetic aperture technique, the resolution can be improved while SNR(signal-to-noise ratio) is decreased.
The pulser 11 is a bipolar pulser which supplies an amplified signal (e.g., +80 or xe2x88x9280 volt) to the transducer array 1 in response to the binary sequence input corresponding to the delay pattern. The voltage output of the pulser 11 having predetermined amplitude is applied to each transducer of the transducer array 1 at a time determined by the delay pattern.
The transducer array 1 includes a number of transducer elements and transmits the ultrasound pulses, in response to the output voltage of the pulser 11, into an object such as a human body. A portion of the transducer array 1 is used for transmission at a time. For example, even if the transducer array 1 includes 128 transducers, only 64 transducers within an aperture transmit the ultrasound at one time.
The transducer array 1 also receives a signal including reflected pulses of the transmitted ultrasound pulse, which is reflected from inside the body.
The TX/RX switch 21 acts as a duplexer for isolating the receiver 31 from the effect of the high voltage output from 10 the pulser 11. The switch 21 connects the transducer array 1 to the pulser 11 during transmission mode and to the receiver 31 during reception mode.
The receiver 31 includes a pre-amplifier for amplifying the received signal, a TGC (time gain compensation) for compensating the attenuation during propagation of the ultrasound and an analog-to-digital converter for converting the amplified received signal to a digital signal.
The beamformer 35 performs the receiving focusing in accordance with the delay pattern from the RX focus delay adjuster 36.
The signal processor 41 performs signal processing such as envelope detection, log compensation to produce a B-mode image signal.
The scan converter 42 converts the B-mode image signal to a signal which may be visualized on a display device (not shown).
Due to the decrease in power of the ultrasound during propagation into highly attenuating medium such as rubber, soft tissue and the like, the short-pulse imaging system may not obtain information for a target object inside the body from which the short pulses are reflected.
Since the medical ultrasound imaging system 100 may cause damage to the body when it increases the peak voltage of the transmitted short pulses, the power of the received signal can not be increased in this way.
On the other hand, a pulse compression technique used in a radar apparatus is capable of improving the SNR of the ultrasound imaging system by increasing the average power instead of by increasing the peak voltage of the transmitted pulse. In the imaging system using the pulse compression technique, a coded long pulse is transmitted to the body instead of the short pulse.
In the medical imaging system 100 using the conventional short pulse, the image resolution in the ultrasound propagation direction depends on the impulse response of the ultrasound transducer used because the short pulse of a high voltage is used. However, in the imaging system using the pulse compression technique, the image resolution is determined by the convolution of the ultrasound transducer and the pulse, because the coded long pulse is used. In the pulse compression technique imaging system, a pulse compressor having a correlator at the ultrasound receiver achieves the effects of the short pulse transmission technique. Accordingly, it is capable of effectively increasing the SNR by transmitting the coded long pulse having a lower voltage than the peak voltage in the short pulse technique.
In the ultrasound imaging system using the coded long pulse, the system performance depends on the code characteristics. In particular, the resultant image quality depends on the relation of the frequency characteristics of a used code and the ultrasound transducer. And the system performance also depends on the pulse compressor implementation or the correlator implementation.
There has been some efforts to apply Golay codes to the long-pulse ultrasound imaging system because the Golay codes haves a characteristic of eliminating side-lobes. However, one of undesirable frequency characteristics of the Golay codes is a wider frequency spectrum than that of the conventional ultrasound transducer. That is, there is some loss in the power of a Golay code at the ultrasound transducer such that the SNR of the system can not reach a desired level.
An objective of the present invention is to provide codes of which frequency characteristics match with the frequency characteristic of the ultrasound transducer and an ultrasound imaging method based on a pulse compression technique using the codes.
Another objective of the present invention is to provide an imaging apparatus for effectively implementing the ultrasound imaging method using the codes.
In accordance with one aspect of the present invention, there is provided an ultrasound imaging method based on the pulse compression technique using Golay codes modified by using a predetermined window function.
In accordance with another aspect of the present invention, there is provided a method and apparatus for transmitting ultrasound pulses of the modified Golay codes, pulse-compressing the reflected signals corresponding the transmitted pulses and performing RX-focusing on the pulse-compressed signals.
The present invention provides an ultrasound imaging method for forming an image of an object using signals reflected from the object after transmitting an ultrasound pulse to the object, comprising the steps of;
(a) transmitting a first set of the ultrasound pulses to the object by applying voltages according to a first code of a pair of modified Golay codes to one or more transducers;
(b) performing pulse compression on a first set of reflected signals of the first set of the ultrasound pulses reflected from the object;
(c) transmitting a second set of the ultrasound pulses to the object by applying the voltages according to a second code of the pair of the modified Golay codes to said one or more transducers;
(d) performing pulse compression on a second set of reflected signals of the second set of the ultrasound pulses reflected from the object;
(e) adding the pulse compressed signals of the first and the second sets of the reflected signals;
(f) producing a receive-focused signal by using the added signal; and
(g) displaying the image according to the receive-focused signal after signal processing.
And the present invention provides an ultrasound imaging apparatus for forming an image of an object using received signals reflected from the object after transmitting an ultrasound pulse to the object, comprising:
means for applying voltages according to a first code of a pair of modified Golay codes to one or more transducers at a first time period and for applying the voltages according to a second code of the pair of the modified Golay codes to said one or more transducers at a second time period;
means for receiving after the first time period a first set of reflected signals of the first set of the ultrasound pulse reflected on the object and receiving after the second time period a second set of reflected signals of the ultrasound pulse reflected on the object;
means for performing pulse compression on the first and second sets of reflected signals to produce a first and a second pulse-compressed signals and adding the first and the second pulse compressed signals;
means for producing a receive-focused signal by using the added signal; and
means for displaying an image according to the receive-focused signal after signal processing.